(1) Field of the Invention
The present invention relates to a semiconductor device having a probe pad of a POE (Pad on element) type in which a pad is formed on semiconductor active elements, and more particularly relates to a connection structure of an electrode pad part of a semiconductor device.
(2) Description of Related Art
The structure of an electrode pad part of a known semiconductor device will be described hereinafter with reference to FIGS. 12 and 13 (see, for example, Japanese Laid-Open Patent Publication No. 5-343466).
FIG. 12 is a cross-sectional view of the principal part of a known semiconductor device showing the structure of an electrode pad part of the known semiconductor device. FIG. 13 is a plan view taken along the line XIII-XIII shown in FIG. 12.
As shown in FIG. 12, a first interconnect layer 2 is formed on a predetermined region of a semiconductor substrate 1. A first interlayer insulating film 3 is formed on the semiconductor substrate 1 and first interconnect layer 2. As shown in FIG. 13, a plurality of first contact holes 3a are formed in a region of the first interlayer insulating film 3 located on the first interconnect layer 2, and a second interconnect layer 4 is formed inside the first contact holes 3a and on the region of the first interlayer insulating film 3 located on the first interconnect layer 2. A second interlayer insulating film 5 is formed on the first interlayer insulating film 3 and the second interconnect layer 4. As shown in FIG. 13, a plurality of second contact holes 5a are formed in a region of the second interlayer insulating film 5 located on the second interconnect layer 4.
A third interconnect layer 6 is formed inside the second contact holes 5a and on a predetermined region of the second interlayer insulating film 5 located on the second interconnect layer 4. The third interconnect layer 6 serves as a pad electrode. A protective film 7 is formed on a predetermined region of the third interconnect layer 6 and on the second interlayer insulating film 5. A pad opening 7a with a desired opening area is formed in a region of the protective film 7 located on the third interconnect layer 6. An unshown bonding wire is connected to the third interconnect layer 6 through the pad opening 7a to allow the input/output of signals from/to an external circuit through the bonding wire.
Since the interlayer insulating films are placed on the first interconnect layer 2 and the second interconnect layer 4 in the electrode pad part having such a structure, the third interconnect layer 6 serving as the electrode pad can be formed in a higher position. In addition, the step height h1 between the top surfaces of the third interconnect layer 6 and protective film 7 can be reduced. In other words, the pad opening 7a, which constitutes a recess of the electrode pad part, can be made shallow. The formation of the shallow pad opening 7a can effectively reduce the occurrence of cracks in the protective film 7. As a result, the reliability of a semiconductor device can be enhanced.
As described above, in the structure of the electrode pad part of the known semiconductor device, the second interlayer insulating film 5 is provided between the second interconnect layer 4 and the third interconnect layer 6, thereby allowing the pad opening 7a of the electrode pad part to be shallow. In this way, cracks that may occur in the protective film 7 are reduced.
However, in the structure of the electrode pad part of the known semiconductor device, a stress caused by a load to an electrode pad during probing and bonding produces a crack in an insulating film formed below the electrode pad. The reason why a crack is produced as described above is that the insulating film located below the electrode pad has a low strength and thus the stress caused by the load to the electrode pad cannot be absorbed by the insulating film located below the electrode pad. When the produced crack reaches an insulating film further below the electrode pad, this damages a semiconductor element below the insulating film.
To be specific, as shown in FIG. 14, when a probe needle 14a comes into contact with the third interconnect layer 6 serving as an electrode pad, that is, when the tip of the probe needle 14a is pressed against the electrode pad with an appropriate needle pressure, the probe needle 14a slides in the horizontal direction 14b and the tip of the probe needle 14a is engaged in the electrode pad. Thus, a low contact electrical resistance can be obtained between the probe needle 14a and the electrode pad. In this case, the stress applied from the probe needle 14a is used for the plastic deformation of a metal constituting the electrode pad. However, as the probe needle 14a comes into contact with the electrode pad again and again, the electrode pad becomes thinner. Then, when the tip of the probe needle 14a reaches the vicinity of the bottom surface of the electrode pad, the stress is applied from the probe needle 14a even to the underlying structure of the electrode pad, thereby producing a crack 14c. Therefore, a leakage current flows along the produced crack 14c, leading to a malfunction in a circuit placed below the electrode pad.